Integrated optic components

ABSTRACT

An integrated optic component comprises a substrate (10) carrying a layer (11) of polymeric material which has an aliphatic or aromatic polymer backbone with bonded sidegroups exhibiting hyperpolarizability. The layer (11) shows a refractive index pattern induced by irradiation with wavelengths within the electronic absorption bands of the sidegroups and within the range of about 230 to 650 nm. The component may be poled so as to be an active component and may be in the form of a ridge guide. Typical sidegroups comprise 4&#39;-amino or 4&#39;-oxy substituted 4-nitrostilbene moieties, or 4&#39;-amino substituted 4-cyanostilbene moieties, or N-substituted 1-(4-aminophenyl)-4-(4-nitrophenyl) buta-1,3-diene moieties.

This application is a divisional of application Ser. No. 07/405,147,filed on Sept. 7, 1989, now U.S. Pat. No. 5,142,605, issued Aug. 25,1992.

This invention relates to integrated optic components.

Passive integrated optic components are well known and comprise asubstrate one surface of which carries a refractive index pattern whichdefines the nature of the component. Typical patterns define waveguidecircuits, waveguide gratings, and optical couplers. In order tofabricate such integrated optic components it has been proposed to applya thin film of suitable material to the surface of a substrate and tophotolithographically process the thin film so as to form the desiredpattern. Other processing techniques proposed include laser beam writingor contact printing or holographic exposure. The materials which havebeen proposed, for example as described in `Light Guiding Structures ofPhotoresist Films` Appl. Phys. Lett. Vol. 20 page 143 et seq and`High-resolution photorefractive polymer for optical recording ofwaveguide gratings` Applied Optics Vol. 25 page 2960 et seq, suffer fromvarious disadvantages such as very long processing times to obtain ausable refractive index pattern or very short retention duration of therefractive index pattern.

Active integrated optic components are also well known being fabricatedin electro-optic material such as lithium niobate whereby the in-usefunction of the component is controllable by an external electricalfield. It has recently been proposed to fabricate active integratedoptic components using a thin film of suitable electro-optic materialcarried by one surface of a substrate, for example as described in`Poled electro-optic waveguide formation in thin film organic media`Appl. Phys. Lett. Vol. 52 page 1031 et seq but the method thereindescribed has the disadvantage that the guide properties andelectro-optic interaction are combined and optimisation for bothefficient guide formation and efficient guide operation under theelectro-optic effect cannot be achieved.

In accordance with the present invention there is provided an integratedoptic component comprising a substrate carrying on at least one surfacethereof a thin film of a polymeric material having the possibility to bemade electro-optic, characterised in that the polymeric material has analiphatic or aromatic polymer backbone with bonded sidegroups exhibitinga hyperpolarizability, and that the material shows a refractive indexpattern induced by irradiation with wavelengths within the electronicabsorption bands of the sidegroups.

The sidegroups may be bonded to the backbone either directly or via aspacer, or the sidegroups may be partly incorporated in the backbone.

The aliphatic or aromatic polymer backbone may take any one of a numberof different forms, for example, polyurethane, polyesters,polyacrylates, polyamides, polyethers or polysiloxanes.

Particular polymeric materials suitable for the integrated opticcomponent according to the invention have sidegroups consisting of asystem having the following structure:

    --R.sub.1 --(φ.sub.i).sub.k --(R.sub.2 =R.sub.3).sub.m --(φ.sub.j).sub.l --(R.sub.4 =R.sub.5).sub.n --R.sub.6

wherein

R₁ =--O--, N--, --S--, or ##STR1## φ_(i) =a cyclic conjugated moiety,e.g., phenyl k=0 through 4 ##STR2## m=0 through 6 φ_(j) =a cyclicconjugated moiety, e.g., phenyl

l=0 through 4 ##STR3## n=0 through 6 ##STR4##

The sidegroups may comprise 4'-amino or 4'-oxy substituted4-nitrostilbene moieties, or 4'-amino substituted 4-cyanostilbenemoieties, or N-substituted 1-(4-amino-phenyl)-4-(4-nitrophenyl)buta-1,3-diene moieties, for example 4'-amino substituted4-nitrostilbene.

The invention is also related to a method of manufacturing an integratedoptic component comprising a substrate carrying on at least one sidethereof a thin layer of a polymeric material having the possibility tobe made electro-optic. This method is characterised in that as polymericmaterial a material is chosen having an aliphatic or aromatic backbonewith bonded sidegroups exhibiting hyperpolarizability, and that intosaid polymeric material a refractive index pattern is induced byselectively exposing said material with irradiation with wavelengthswithin the electronic absorption bands of the sidegroups.

By virtue of the present invention the processing time for obtainingusable refractive index patterns is substantially reduced, being of theorder of a few tens of minutes (e.g. 50 minutes) and the duration of therefractive index pattern is at least in the order of years. Therefractive index of the polymeric material is sensitive to irradiationin the waveband 230 to 650 nm, displays increased sensitivity withrespect to refractive index changes in comparison to the known materials(by a factor of 10) and may in addition have electro-optic properties.Accordingly fabrication of the component of the present invention isachieved in a manner similar to that previously used for passivecomponents but, with poling, produces an active component. Bothefficient guide formation and efficient guide operation are therebyachieved in an active component.

The selective exposure of the polymeric material to the irradiation maybe performed by a selective "writing" procedure, e.g., by a focussedlaser beam or by holographic means. According to a particular method ofmanufacturing the selective exposure occurs by means of a mask havingparts which are transparent and parts which are opaque to the appliedirradiation.

Another aspect of the present invention comprises the fabrication of anactive integrated optic component including the steps of successivelyforming on a substrate a first layer of electrode material, a firstlayer of buffer material, a layer of the polymeric material, a secondlayer of buffer material, and a second layer of electrode material,etching the second layer of electrode material to form an electrode ofpredetermined pattern, and directing irradiation with wavelengths withinthe electronic absorption bands of the sidegroups onto the exposedsurface of the component to cause a change of the refractive index ofthose areas of the polymeric material which are not covered by theelectrode of predetermined pattern, the buffer material of at least thesecond buffer material layer being non-opaque to the directedirradiation.

The present invention also comprises the fabrication of a passiveintegrated optic component, including the steps of forming on asubstrate a layer of polymeric material having an aliphatic or aromaticpolymer backbone with bonded sidegroups exhibiting ahyperpolarizability, irradiating the polymeric material partway over itsthickness with irradiation with wavelengths within the electronicabsorption bands of the sidegroups confined to a first predeterminedpattern so as to cause a change of the refractive index of those areasof the polymeric material which are so exposed, applying a thin film ofsaid polymeric material dissolved in a solvent to the surface of thepolymeric material exposed to said irradiation for a measured timeinterval so as to dissolve the exposed area of index-changed material,and thereafter washing off the thin film to leave a relief surfacestructure which is free of index-changed material on the polymericmaterial.

By virtue of the ease of manufacture of integrated optical componentsusing the aforesaid polymeric material and achieving patterning throughirradiation exposure modification of an existing pattern is easilyachieved at a local site by further irradiation at the site utilisingfor example focussed radiation from a laser source. This is particularlyadvantageous in the fabrication of large arrays of integrated opticelements, such as switches, where each element in the array is intendedfor example to have identical characteristics but due to inhomogeneitiessome elements display undesired characteristics. After defining thecomponent array by selective exposure according to the method of thepresent invention, characteristics of individual components withundesired tested to identify those components with undesiredcharacteristics. Components with undesired characteristics can then betuned by local application of further irradiation to provide the desiredcharacteristics. Localised trimming or tuning of these elements byapplication of further irradiation is easily performed to achieve therequired characteristics.

Preferably the irradiation is applied to the polymeric material when thelatter is at an elevated temperature since it has been discovered thatthe processing time for obtaining a particular refractive index patterndecreases as temperature increases.

It will be appreciated that the term `hyperpolarisability` is known tothose skilled in the art of optoelectronic materials, see for example anarticle by Le Barny et al in SPIE Vol. 682 Molecular and PolymericOptoelectronic Materials: Fundamentals and Applications (1986) pp 56-64.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 illustrates a substrate supporting a layer of polymeric materialin accordance with the present invention;

FIG. 2 shows selective irradiation of the FIG. 1 structure;

FIG. 3 illustrates a typical integrated optic component produced fromthe FIG. 2 process;

FIG. 4 shows an alternative form of component;

FIG. 5 is a graph illustrating a processing function used in manufactureof the FIG. 4 component;

FIG. 6 shows a further component derived from the FIG. 4 component;

FIG. 7 illustrates a still further form of integrated optic component;

FIGS. 8 and 9 illustrate process steps in the manufacture of the FIG. 7component; and

FIG. 10 illustrates a typical irradiation absorption spectrum for apolymeric material used in accordance with the present invention.

As is illustrated in FIG. 1 a substrate 10 having a refractive index ofvalue n₁ is provided on its upper surface with a thin film or layer ofpolymer material 11. The material 11 has a polyurethane backbone towhich side groups are chemically bonded, the side groups consisting of4'-amino or 4'-oxy substituted 4 nitrostilbene, namely 4-nitro-4'aminostilbene or 4-nitro-4' oxystilbene. The side groups of the polymermaterial confer to the material a refractive index sensitivity so thatprior to suitable irradiation the material is of uniform refractiveindex value n₂ but upon irradiation of selected areas of the polymermaterial with radiation in the 230-650 nm waveband the exposed areas areleft with a refractive index of value n₃ (which is less than n₂). In theparticular example schematically illustrated in FIG. 2 the irradiatedareas are selected by a mask 12 having non-transmitting lines 13 thereonso that the radiation is transmitted by the mask between the lines 13and thereby (as shown in FIG. 3) produces lines 14 on the polymermaterial 11 of index value n₃ separated by lines 15 of index value n₂all carried by the substrate of index value n₁ and n₂ >n₃ >n₁.

The polymer material which has been described exhibits refractive indexsensitivity in the wavelength range 280-450 nm which is particularlyuseful since it enables exposure by UV sources, for instance in maskaligners thereby facilitating integrated optic components to befabricated with excellent reproducability, with a slightly-graded orsharp transition in the index profile at the n₂ /n₃ interface whichalong the length of the transition is smooth and uniform from guide toguide. The waveguides thus formed are of relatively low loss per unitlength. The processing time for irradiation is of the order of a fewtens of minutes and because the side groups of the polymer material arebonded to the backbone, it has been found that the irradiated pattern isstable insofar as it is effectively permanently recorded.

Furthermore the side groups of the polymer materials described displayelectro-optic characteristics as a result of which the integrated opticcomponents, after poling, can be electrically operated during use bymeans of electrodes 16 deposited thereon (FIG. 3), for example to forman electro-optic modulator or other active component.

FIGS. 1 to 3 are schematic and indicate that the irradiation exposure isapplied for a sufficient duration to cause conversion of the whole depthof the n₂ index material to n₃ index but for most integrated opticapplications it is sufficient to expose for a duration sufficient toeffect only a predetermined depth of n₃ index within the body of n₂index as is illustrated in FIG. 4. For any given depth D of n₃ index ithas been found that the processing time or exposure time reduces astemperature of substrate 10 and material 11 increases. FIG. 5illustrates comparative graphs of exposure time and depth for twotemperatures, 20° C. and 130° C.

We have also found that the polymeric material 11 behaves like aphotoresist in that after patterning by irradiation exposure asexplained with reference to FIG. 4 the n₂ /n₃ index pattern can beconverted to a relief pattern of n₂ index by dissolving out the n₃ indexareas simply by coating the n₂ /n₃ index pattern with a developer in theform of a thin uniform film of the polymeric material dissolved in asuitable solvent such as cyclopentanone. After a period of timedependant upon the depth of the n₃ index areas the excess developer isremoved for example by spinning to leave a layer of the n₂ indexmaterial with a relief pattern as shown in FIG. 6. This arises becausethe irradiation exposure in creating the n₃ areas modifies thesolubility of these areas so that they are substantially more solublethan the un-exposed n₂ areas. It will be understood that the resultantintegrated optic component can be used as a raised ridge guide, or as awaveguide taper structure or as a surface relief hologram.

As has been indicated with reference to FIG. 3 if the component is to beused as an active optic component it requires electrodes 16. Theelectrodes 16 of FIG. 3 are essentially co-planar with the lines 15 ofn₂ index and they interfere with the process of poling the material 11which, of course, is necessary to align the side groups of the polymericmaterial so that its inherent electro-optic capabilities are usable. Apreferred electrode arrangement is the plane-parallel format illustratedin FIG. 7 and which is manufactured as indicated in FIGS. 8 and 9.

In FIG. 7 the material 11 suitably index patterned as will be explainedis separated from upper and lower 10 electrodes 20, 21, by inactivebuffer layers 22A, 22B. This is achieved by constructing on thesubstrate 10 the layered structure shown in FIG. 8 with the upperelectrode 20 initially of the same area as the lower electrode 21. Thematerial 11 is first poled by applying a poling voltage across theelectrodes 20, 21, with the material 11 at elevated temperature. Thisaligns the side groups of the material 11. Next, with the poling voltageremoved, the upper electrode 20 is patterned by dissolving out theunrequired metal. Buffer layer 22A acts as a chemical barrier duringthis process and the structure has the form shown in FIG. 9. Next,irradiation is applied over the whole area of the top surface of theFIG. 9 structure which results in conversion of the n₁ index areasexposed to the irradiation to n₃ index areas and the only non-exposed n₂area is that underlying the patterned electrode 20 as shown in FIG. 7.Thus the n₂ index area is automatically self aligned with the electrode20. The electrode 20 is made of metal, for example gold, and the bufferlayer 22A is non-opaque to the radiation, being made for example ofpolyurethane. The step of locally poling the polyermic material byapplication of a poling voltage between the first and second electrodematerial layers may also be performed after the etching of the secondlayer of electrode material.

With the self aligning fabrication process described with reference toFIGS. 8 and 9 production of integrated electric and optic components isgreatly simplified by removing failures due to misalignment of thepatterned electrode and the n₂ index area.

In a modification of the FIG. 7 component electrode 21 is omitted as isthe step of poling the polymeric material 11. In this case the resultantcomponent is rendered thermoactive by current flow along the topconductor 20.

The irradiation which has been referred to hereinbefore is preferably inthe range between about 250 to 570 nm. The main effects however areapparent in the range of about 280 to 450 nm. In particular when thepolymer contains side groups of 4'-oxy substituted 4-nitrostilbene,irradiation at about 373 nm results in a maximum sensitivity torefractive index changes. When the polymer contains side groups of4'-amino substituted 4-nitrostilbene, irradiation at about 429 nm hasprovided excellent results both with respect to refractive index changesand low optical losses at the operating wavelength. By way of exampleonly, FIG. 10 illustrates the absorption spectrum of one tested sampleof the amino substituted polymeric material illustrating that absorptionpeaks occur at 300 nm and 430 nm which are primarily due to theelectronic absorption bands of the sidegroups of the material.

In the case of a polymer of the 4'-amino substituted 4-cyanostilbenetype irradiation at about 384 nm wavelength appeared to result in amaximum sensitivity to refractive index changes, whereas for a polymerwith sidegroups comprising N-substituted1-(4-aminophenyl)-4-(4-nitrophenyl) buta-1,3-diene moieties the maximumsensitivity to refractive index changes appeared at a wavelength of 442nm. By way of example, tested samples of the amino substituted polymericmaterial displayed for a transmission wavelength in the range 1.3 μm to1.5 μm an index decrease of 0.03, and for a transmission wavelength of0.6328 μm an index decrease of 0.09. Channel waveguides have been madefrom 4'-amino substituted 4-nitrostilbene which showed losses below 1dB/cm.

We claim:
 1. A method of manufacturing an integrated optic componentcomprising:providing a substrate carrying on at least one side thereof athin layer of polymeric material capable of being made electro-optic,wherein said polymeric material is a material having an aliphatic oraromatic backbone with bonded sidegroups exhibiting hyperpolarizability;and selectively exposing portions of said polymeric material toirradiation with wavelengths within the electronic absorption bands ofthe sidegroups to induce into said polymeric material a predeterminedrefractive index pattern.
 2. A method as claimed in claim 1, furthercomprising the step of providing on said polymeric material prior tosaid exposing step a mask having parts which are transparent and partswhich are opaque to the applied irradiation.
 3. A method as claimed inany one of claims 1, wherein the irradiation is applied to the polymericmaterial when the latter is at an elevated temperature.
 4. A method asclaimed in claim 1, herein the sidegroups comprise 4'-amino or 4'-oxysubstituted 4-nitrostilbene moieties, 4'-amino substituted4-cyanostilbene moieties, or N-substituted1-(4-aminophenyl)-4-(4-nitrophenyl)buta-1,3-diene moieties.
 5. A methodas claimed in claim 1, wherein the irradiation has a wavelength withinthe range of about 230 to 650 nm.
 6. A method as claimed in claim 5,wherein the irradiation has a wavelength within the range of about 250to 570 nm.
 7. A method as claimed in claim 6, wherein the irradiationhas a wavelength within the range of about 280 to 450 nm.